Electric cable comprising a foamed polyolefine insulation and manufacturing process thereof

ABSTRACT

A process for manufacturing an electric cable including at least one core including a conductor and an insulating coating surrounding the conductor includes the steps of: providing a polyolefin material, a silane-based cross-linking system and a foaming system including at least one exothermic foaming agent in an amount of 0.1% to 0.5% by weight with respect to the total weight of the polyolefin material; forming a blend with the polyolefin material, the silane-based cross-linking system and the foaming system; and extruding the blend on the conductor to form the insulating coating. An electric cable includes at least one core consisting of a conductor and an insulating coating surrounding the conductor and in contact therewith, the insulating coating consisting of a layer of expanded, silane-cross-linked polyolefin material having an expansion degree of 3% to 40%.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/EP2005/013866, filed Dec. 22, 2005, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an electric cable.

Furthermore, the present invention relates to a manufacturing process ofsaid electric cable.

PRIOR ART

Cables for power transmission are generally provided with a metallicconductor which is surrounded by an insulating coating.

A power cable can be provided with a sheath in a radially externalposition with respect to the insulating layer. Said is sheath isprovided for protecting the cable against mechanical damages.

U.S. Pat. No. 4,789,589 relates to an insulated electrical conductorwire, wherein the insulation surrounding the conductor wire comprises aninner layer of a polyolefin compound and of cellular construction, andan outer layer of a non-cured and non-curable polyvinylchloride.

WO 03/088274 relates to a cable with an insulating coating comprising atleast two insulating layers so that, in a radial direction from theinside towards the outside of the cable, the insulating coatingcomprises at least one insulating layer made of a non-expanded polymericmaterial and at least one insulating layer made of an expanded polymericmaterial. In fact, an expanded insulating layer shows discontinuities(i.e., voids within the polymeric material, said voids being filled withair or gas) and could not work properly in the space surrounding theconductor where the electrical field is most relevant.

As reported, for example, by U.S. Pat. No. 4,591,606, cross-linkedpolyolefin foam is produced by using chemical foaming agents, such asazodicarbonamide, which decompose on being heated and generate gaseousnitrogen. The cross-linking is usually achieved by the aid of a radicalformer, such as dicumylperoxide. The cross-linking reaction is alsoachieved with the aid of heat. Cross-linked polyethylene foammanufacturing processes have also been developed, but in this casecross-linking is accomplished with the aid of irradiation. The productsof such process have very low densities, thus no applications requiringstrength and rigidity can be contemplated. When an organic peroxide isused as a cross-linking agent, control of the process is difficultbecause foaming and cross-linking process, are bothtemperature-dependent.

U.S. Pat. No. 3,098,831 relates to cross-linked and expandedpolyethylene material useful, inter alia, as electrical insulation. Saidpolyethylene material is said to have a density of not more than 0.32g/cm³ (20 pounds per cubic foot). Examples are provided withpolyethylene having an expansion degree of 90-95%. The expandedpolyethylene is prepared by subjecting cross-linked polyethylenecontaining a rubber foaming agent to an elevated temperature at whichthe foaming agent is decomposed and thus causes the polyethylene toexpand. The polyethylene starting material may be cross-linked, e.g., byan organic peroxide, the amount of cross-lining agent generally varyingfrom 0.002 to 0.01 mol per 100 grams of polyethylene. Among the foamingagents, azodicarbonamide is exemplified, and about 2 to 15 parts byweight of foaming agent, based on 100 parts of the polyethylenematerial, are employed.

Generally, a cable for building wiring and/or industrial applicationsshould be installed within walls, and the installation process requiresthat the cable passes through walls restrictions or, more frequently,that the cable is pulled through conduits, wherein the cable ispermanently confined.

In order to be correctly installed with simple and quick operations, acable needs to be particularly flexible so that it can be inserted intothe wall passages and/or wall conduits and follow the bends of theinstallation path without being damaged.

During customer installation, due to the tortuosity of the installationpath and to friction during the pulling operation, the cables forbuilding wiring are generally subjected to tearing or scraping againstrough edges and/or surfaces.

Increasing the flexibility of an electric cable can allow to reduce thedamages caused by said tearing or scraping actions. As disclosed, forexample, in WO 03/088274 cited above, the flexibility of the cable canbe advantageously increased by providing the cable with an expandedinsulating layer, with favorable results in the installation processthereof.

An increased flexibility can be provided by the expanded insulatinglayer thanks to the “spongy” nature of the material. In particular, theflexibility of a cable can be maximized when the insulating layerconsists of a single layer of expanded material.

In addition, the presence of an expanded coating in a cable decreasesthe cable weight with advantages in the transport and installationthereof.

Nevertheless, an expanded insulating layer could give rise to problemssuch as:

-   -   when in contact with the conductor the discontinuities of an        expanded material could impair the insulating properties of the        layer;    -   the expanded material of the insulating coating should have an        expansion degree high enough to provide the desired flexibility,        but not such to unsuitably weaken the coating from the        mechanical point of view.

Another important aspect which is required to be satisfied by a cable isa simple and quick peeling-off of the cable.

The peeling-off property of a cable, for example for building wiring, isa widely felt request of the market since the peeling-off of a cable isan operation which is manually performed by the technical staff. Forthis reason, said operation is required to be easy and quick to beperformed by the operator, taking also into account that it isfrequently carried out in narrow spaces and rather uncomfortableconditions.

Typically, a cable sheath is made of a mixture based on polyvinylchloride (PVC) and comprising, inter alia, a plasticizer. Theplasticizer is prone to migrate out of the PVC sheath into theinsulating layer altering the composition thereof. In the course ofaccelerated ageing test, the Applicant has observed that this effect issignificant in case of unexpanded insulating layer. As a consequence thecomposition has impaired electrical (insulating) properties, in view ofthe polar nature of the plasticizer, weaken mechanical characteristics,and can bring about premature ageing of the cable.

SUMMARY OF THE INVENTION

The Applicant perceived that an expanded polyolefin material could beadvantageous as insulating layer for a cable when the polyolefinmaterial is both expanded and cross-linked. The co-existingcross-linking and expansion provide a polyolefin material with improvedflexibility and ease of peeling-off without impairing the mechanicalproperties of the layer formed therewith.

The Applicant has observed that if expanding and cross-linking apolyolefin is attempted, the expansion degree cannot in general becontrolled, being either excessive or insufficient.

However, within the present invention the Applicant has found that aproperly expanded and cross-linked insulating layer can be obtained by asilane-based cross-linking system and an exothermic foaming agent. Theso-obtained insulating layer has an expansion degree advantageous toafford the cable with the above-mentioned features.

In particular, the Applicant has found that a polymerexpanded/cross-linked insulating layer improves the ageing stability ofa sheathed cable.

Such result is believed to be due to the fact that such insulating layerhas a better compatibility with respect to the sheath materials.

DEFINITIONS

For the purpose of the present description and of the claims thatfollow, except where otherwise indicated, all numbers expressingamounts, quantities, percentages, and so forth, are to be understood asbeing modified in all instances by the term “about”. Also, all rangesinclude any combination of the maximum and minimum points disclosed andinclude any intermediate ranges therein, which may or may not bespecifically enumerated herein.

In the present description the expression “cable core” indicates astructure comprising at least one conductor and a respective electricinsulating coating arranged in a position radially external to saidconductor.

For the purposes of the present description, the expression “unipolarcable” means a cable provided with a single core as defined above, whilethe expression “multipolar cable” means a cable provided with at leastone pair of said cores. In greater detail, when a multipolar cable has anumber of cores equal to two, said cable is technically defined as“bipolar cable”, if there are three cores, said cable is known as“tripolar cable”, and so on.

In the present description the term “peeling-off of a cable” is used toindicate the removal of all the cable layers which are radially externalto the conductor so that it results uncoated to be electricallyconnected to a conductor of a further cable or to an electricalapparatus, for example.

In the present description, the expression “low voltage” means a voltageof less than about 1 kV.

In the present description and in the subsequent claims, as “conductor”it is meant a conducting element of elongated shape and preferably of ametallic material, e.g. aluminium or copper.

As “insulating coating” or “insulating layer” it is meant a coating orlayer made of a material having an insulation constant (k_(i)) greaterthan 0.0367 MOhm km (as from IEC 60502).

In the present description and claims, as “silane-crosslinked” it ismeant a polyolefin material having siloxane bonds (—Si—O—Si—) as thecross-linking element.

In the present description and claims, as “expanded polyolefin material”it is meant a material with a percentage of free space inside thematerial, i.e. a space not occupied by the polymeric material, but bygas or air, said percentage being expressed by the “expansion degree”(G), defined as follows:

$G = {\left( \frac{d_{0} - d_{e}}{d_{0}} \right) \times 100}$wherein d₀ is the density of the unexpanded polymer and d_(e) is theapparent density measured on the expanded polymer.

The apparent density is measured according to the Italian standardregulation CEI EN 60811-1-3:2001-06.

In the present description and claims, the term “sheath” is intended toidentify a protective outer layer of the cable having the function ofprotecting the latter from accidental impacts or abrasion. From theforegoing, according to the term mentioned above, the cable sheath isnot required to provide the cable with specific electrical insulatingproperties.

In the present description and claims as “silane-based cross-linkingsystem” it is meant a compound or a mixture of compounds comprising atleast one organic silane.

In the present description and claims as “foaming system” it is meant acompound or mixture of compounds comprising one ore more foaming agents,of which at least one is an exothermic foaming agent.

In the present description and claims, as “endothermic foaming agent” ismeant a compound or a mixture of compounds which is thermally unstableand causes heat to be absorbed while generating gas and heat at apredetermined temperature.

In the present description and claims, as “exothermic foaming agent” ismeant a compound or a mixture of compounds which is thermally unstableand decompose to yield gas and heat at a predetermined temperature.

In the present description and claims, as “draw down ratio” it is meantthe ratio of the thickness of the extruder die opening to the finalthickness of the extruded product.

In a first aspect, the present invention relates to a process formanufacturing an electric cable comprising at least one core comprisinga conductor and an insulating coating surrounding said conductor, saidprocess comprising the steps of:

-   -   providing a polyolefin material, a silane-based cross-linking        system and a foaming system comprising at least one exothermic        foaming agent in an amount of from 0.1% to 0.5% by weight with        respect to the total weight of the polyolefin material;    -   forming a blend with the polyolefin material, the silane-based        cross-linking system and the foaming system;    -   extruding the blend on the conductor to form the insulating        coating.

As “polyolefin material” it is meant a polymer selected from the groupcomprising: polyolefins, copolymers of various olefins,olefins/unsaturated esters copolymers, polyesters, and mixtures thereof.Preferably, said polyolefin material is: polyethylene (PE), inparticular low-density PE (LDPE), medium-density PE (MDPE), high-densityPE (HDPE) and linear low-density PE (LLDPE); ethylene-propyleneelastomeric copolymers (EPM) or ethylene-propylene-diene terpolymers(EPDM); ethylene/vinyl ester copolymers, for example ethylene/vinylacetate (EVA); ethylene/acrylate copolymers; ethylene/α-olefinthermoplastic copolymers; and their copolymers or mechanical blends.

More preferred according to the invention is a polyolefin materialselected from polyethylene (PE), in particular low-density PE (LDPE),medium-density PE (MDPE), high-density PE (HDPE) and linear low-densityPE (LLDPE), more preferably LLDPE, optionally in blend with EPDM orolefin copolymer.

When the polyolefin material of the invention is a blend of apolyethylene material and a copolymer material, the latter isadvantageously present in an amount of from 5 phr to 30 phr.

Preferred silanes that can be used are the (C₁-C₄)alkyloxy silanes withat least one double bond, and in particular vinyl- oracryl-(C₁-C₄)alkyloxy silanes; compounds suitable for the purpose can beγ-methacryloxy-propyltrimethoxy silane, vinyltrimethoxysilane,vinyltriethoxysilane, vinyldimethoxyethoxysilane,vinyltris-(2-methoxyethoxy) silane, and mixtures thereof.

The silane-based cross-linking system for the process of the inventioncomprises at least one peroxide. Preferably, peroxides that can beadvantageously used are di(terbutylperoxypropyl-(2)-benzene, dicumylperoxide, di-terbutyl peroxide, benzoyl peroxide, ter-butylcumylperoxide, 1,1-di(ter-butylperoxy)-3,3,5-trimethyl-cyclohexane,2,5-bis(terbutylperoxy)-2,5-dimethylhexane,2,5-bis(terbutylperoxy)-2,5-dimethylhexineterbutylperoxy-3,5,5-trimethylhexanoate, ethyl3,3-di(terbutylperoxy)butyrate, butyl-4,4-di(terbutylperoxy)valerate,and terbutylperoxybenzoate.

Preferably, the silane-based cross-linking system for the process of theinvention comprises at least one cross-linking catalyst, which is chosenfrom those known in the art; preferably, it is convenient to use anorganic titanate or a metallic carboxylate. Dibutyltin dilaurate (DBTL)is especially preferred.

Advantageously, the amount of silane cross-linking system is such toprovide the blend with from 0.003 to 0.015 mol of silane per 100 gramsof polyolefin material. Preferably the amount of silane is of from 0.006to 0.010 mol of silane per 100 grams of polyolefin material.

Optionally the foaming system of the present process comprises at leastone endothermic foaming agent, preferably in an amount equal to or lowerthan 20% by weight with respect to the total weight of the polyolefinmaterial.

Advantageously, the exothermic foaming agent for the process of theinvention is an azo compound such as azodicarbonamide,azobisisobutyronitrile, and diazoaminobenzene. Preferably, theexothermic foaming agent is azodicarbonamide.

Preferably, the exothermic foaming agent is in an amount of from 0.15%to 0.24% by weight with respect to the total weight of the polyolefinmaterial.

Advantageously the foaming system is added to the polyolefinic materialas a masterbatch comprising a polymer material, preferably, an ethylenehomopolymer or copolymer such as ethylene/vinyl acetate copolymer (EVA),ethylene-propylene copolymer (EPR) and ethylene/butyl acrylate copolymer(EBA). Said masterbatch comprises an amount of foaming agent (exothermicand, in case, endothermic) of from 1% by weight to 80% by weight,preferably of from 5% by weight to 50% by weight, more preferably offrom 10% by weight to 40% by weight, with respect to the total weight ofthe polymer material.

Advantageously, the foaming system further comprises at least oneactivator (a.k.a. kicker). Preferably, suitable activators for thefoaming system of the invention are transition metal compounds.

Optionally, the foaming system of the process of the invention furthercomprises at least one nucleating agent. Preferably the nucleating agentis an active nucleator.

Advantageously, the process of the present invention is carried out in asingle screw extruder.

Preferably, the step of extruding the blend on the cable conductor forproviding such conductor of an insulating layer comprises the steps of

-   -   feeding said conductor to an extruding machine;    -   depositing the insulating layer by extrusion.

Advantageously, the step of extruding the blend is effected by means ofa die with a reduced diameter, according to the “draw down ratio” (DDR)lower than 1, preferably lower than 0.9, more preferably lower than 0.8.

Optionally, the manufacturing process according to the invention furthercomprises the step of providing a sheath layer in a radiallycircumferential external position with respect to the at least oneconductor coated with the relevant insulating layer. Such a step iscarried out by extrusion.

In another aspect the present invention relates to an electric cablecomprising at least one core consisting of a conductor and an insulatingcoating surrounding said conductor and in contact therewith, saidinsulating coating consisting essentially of a layer of expanded,silane-crosslinked polyolefin material having an expansion degree offrom 3% to 40%.

Preferably, the electric cable of the invention has three cores asdescribed above.

The electric cable according to the invention is preferably a lowvoltage cable.

As “polyolefin material” it is meant a polymer selected from the groupcomprising: polyolefins, copolymers of various olefins,olefins/unsaturated esters copolymers, polyesters, and mixtures thereof.Preferably, said polyolefin material is: polyethylene (PE), inparticular low-density PE (LDPE), medium-density PE (MDPE), high-densityPE (HDPE) and linear low-density PE (LLDPE); ethylene-propyleneelastomeric copolymers (EPM) or ethylene-propylene-diene terpolymers(EPDM); ethylene/vinyl ester copolymers, for example ethylene/vinylacetate (EVA); ethylene/acrylate copolymers; ethylene/α-olefinthermoplastic copolymers; and their copolymers or mechanical blends.

More preferred according to the invention is a polyolefin materialselected from polyethylene (PE), in particular low-density PE (LDPE),medium-density PE (MDPE), high-density PE (HDPE) and linear low-densityPE (LLDPE), more preferably LLDPE, optionally in blend with EPDM orolefin copolymer.

When the polyolefin material of the invention is a blend of apolyethylene material and a copolymer material, the latter isadvantageously present in an amount of from 5 phr to 30 phr.

More preferably, the insulating coating for the cable of the inventionhas an expansion degree of from 5% to 30%, even more preferably of from10% to 25%.

Advantageously the insulating coating of the cable of the inventionshows an expansion characterized by a specific average cell diameter.

In particular, the insulating coating of the cable of the inventionadvantageously has an average cell diameter equal to or lower than 300μm, preferably equal to or lower than 100 μm.

Advantageously, the insulating coating of the invention is not expandedin a circumferential portion in contact with and/or in the vicinity ofthe conductor, i.e. substantially no cells are present therein.

Preferably, the cable according to the present invention is providedwith a sheath layer, in radially external position with respect to theinsulating layer, preferably in contact thereto.

Preferably, said sheath layer is made of a compound comprising polyvinylchloride (PVC), a filler, such as chalk, a plasticizer, e.g. octyl,nonyl or decyl phthalate, and additives.

In a further aspect, the present invention relates to a method forimproving the ageing stability of a cable comprising a conductor, aninsulating layer and a sheath, wherein the said insulating coatingcomprises a silane-crosslinked polyolefin material having an expansiondegree of from 3% to 40%.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages will become clearer in the lightof the following description of some preferred embodiments of thepresent invention.

The following description makes reference to the accompanying drawings,in which:

FIG. 1 shows a cross right section of an example of a cable according tothe present invention;

FIG. 2 is a photograph of a sample of insulating layer from comparativecable 17;

FIG. 3 is a photograph of a sample of insulating layer from cable 19according to the invention;

FIG. 4 is a photograph of a sample of insulating layer from cable 20according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the cross section of a cable according to the invention forpower transmission at low voltage.

Cable 10 is of the tripolar type (with three cores) and comprises threeconductors 1 each covered by an expanded and cross-linked polymerinsulating coating 2. The three conductors 1 with the relevantinsulating coatings are encircled by a sheath 3.

The insulating constant k_(i) of the electrical insulating layer 2 issuch that the required electric insulating properties are compatiblewith the standards (e.g. IEC 60502 or other equivalent thereto). Forinstance, the electrical insulating layer 2 has an insulating constantk_(i) equal to or greater than 3.67 MOhm km at 90° C.

The expansion degree of the insulating layer for the cable of theinvention is of from 3% to 40%. In particular, the Applicant observedthat an expansion degree lower than 3% does not provide the cable withappreciable advantages in term of flexibility and weight reduction. Onthe other side when the expansion degree is higher than 40%, themechanical characteristics of the cable, e.g. the tensile strength areimpaired to an extent unacceptable for the installation requirement.

FIG. 1 shows only one of the possible embodiments of cables in which thepresent invention can be advantageously employed. Therefore, anysuitable modifications can be made to the embodiments mentioned abovesuch as, for example, the use of cables of the multipolar type orconductors of sectorial cross section.

According to the present invention, in order to confer to the insulatingcoating a suitable mechanical resistance without decreasing theflexibility of the cable, the expanded polyolefin material of thereof isobtained from a polyolefin material that, before expansion, has aflexural modulus at room temperature, measured according to ASTMstandard D790-86, comprised between 50 MPa and 1,000 MPa. Preferably,said flexural modulus at room temperature is not greater than 600 MPa,more preferably it is comprised between 100 MPa and 600 MPa.

For example, the cable of FIG. 1 can be produced by a process carriedout in an extrusion apparatus with a single screw extruder having adiameter of from 60 to 175 mm, and a length about 20 D to 30 D, thesecharacteristics being selected in view of the diameter of the cable tobe obtained and/or of the desired speed production.

Suitably, the screw can be a single flight screw, with the optionalpresence of barrier flight in the transition zone; preferably no mixerdevice is adopted along the screw.

The extrusion apparatus is advantageously fed by a multi componentdosing system of gravimetric type or, preferably, of volumetric type.The dosing system can feed the ingredients (polyolefin material,silane-based cross-linking system and foaming system).

If a colored cable is desired (either wholly colored or provided with acolored skin coating), a pigment master batch can used.

The above-mentioned ingredients are advantageously fed to the feedingthroat of the extruder in pellet form and dosed in the desiredpercentage through a gravimetric or volumetric control system. Apreliminary mixing of the ingredients, off-line or in the hopper abovethe feed throat, can advantageously improve the dispersion of componentsand the final product quality.

Optionally, the cross-linking system, typically available in liquidstate, is introduced in the extruder by injecting it at the bottom ofextruder hopper (top of feeding throat) at low pressure (1 bar); thepercentage of cross-linking system introduced can be gravimetrically orvolumetrically checked.

For example, the above listed ingredients are fed in the extruderthroat, heated, melted and mixed by the screw along the extruder andfinally metered to the extrusion crosshead.

Along the extruder, the grafting of silane groups to polymeric chains ischemically activated and the cross-linking process starts.

The expansion of the polyolefin material for the insulating coating ofthe invention is accomplished by means of a specific foaming agent. Suchfoaming agent is advantageously selected from the group of theexothermic foaming agent, in particular of the azo compounds such asazodicarbonamide, azobisisobutyronitrile, and diazoaminobenzene. The azocompounds are preferred foaming agent by virtue of their chemicalinertia with respect to reactants employed in the preparation of theinsulating coating, especially with respect to the cross-linking system.

The foaming system is blended with the other ingredients and start todecompose at a predetermined temperature. After reaction, the gasgenerated by the foaming system remains dispersed inside the blend.

The blend, after passing through the filtration unit, is fed, forexample, to a crosshead where it is distributed around the conductor inan orthogonal configuration with respect to the extruder. In the diezone, the conductor is coated by the blend and, after the dies when thepressure is released, the expansion of the blend starts; After a lengthof, e.g., 1 m where the coated conductor is exposed to ambient, the sameis plunged in the cooling through, where it is subject to cooling byturbulent water or other similar cooling liquid. The cooling through canbe of single pass or multi pass type.

The expansion phase of the extruded insulating layer is stopped as soonas the melt is cooled down, so it should happen in a short time.

At the end of the cooling unit the insulated conductor is dried, forexample, by use of air jet system or heating, and subsequently taken upon drums.

At this stage, the cross-linking of the insulating coating goes onoptionally with the aid of water and temperature; the time delay forcompleting of the cross-linking phase can be reduced by placing a drumwith the insulated conductor inside a curing room (sauna).

The step of extruding the blend can be effected by means of a die with areduced diameter, according to the “draw down ratio” (DDR), in order toincrease the compression on the melted compound and obtain an expansionwith improved regularity and dimension of the cells.

As from above, in the present process the exothermic foaming agent is inan amount of from 0.1% to 0.5% by weight with respect to the totalweight of the polyolefin material. Amounts lower than 0.1% by weightyield negligible expansion degrees of the polyolefin material. On theother side, as it will be shown in the accompanying examples, amountshigher than 0.5% by weight yield expansion degrees so high to impair themechanical characteristics of the products.

The foaming system of the invention can further comprise at least oneactivator, for example zinc-, cadmium- or lead-compounds (oxides, salts,usually of a fatty acid, or other organometallic compounds) amines,amides and glycols.

The foaming system of the process of the invention can further compriseat least one nucleating agent. The nucleating agent provides nucleatingsites where the physical foaming agent will come out of solution duringfoam expansion; a nucleating site means a starting point from where thefoam cells start growing. If a nucleating agent can provide a highernumber of nucleating sites then more cells are formed and the averagecell size will be smaller.

Two types of nucleating agents can be used in the process of theinvention, inactive (or passive) and active nucleators. Inactivenucleators include solid materials with fine particle size such as talc,clay, diatomaceous earth, calcium carbonate, magnesium oxide and silica.These materials function as nucleators by providing an interruption inthe system when the foaming agent comes out of solution to start abubble. The efficiency of these materials is effected by the shape andsize of the particle. Chemical foaming agents, materials which generategas upon decomposition, e.g. azodicarbonamide, can also act as activenucleators. The nucleation of direct gassed systems with chemicalfoaming agents is called “active nucleation”. Active nucleators arepreferable as more efficient and providing smaller and more uniformcells versus inactive nucleators.

The amount of silane cross-linking system is such to provide the blendwith from 0.003 to 0.015 mol of silane per 100 grams of polyolefinmaterial. An amount of silane lower than 0.003 mol of silane does notprovide a sufficient cross-linking of the polyolefin material, while anamount higher than 0.015 mol, besides being in large excess, can causescrew slipping in the extruder.

EXAMPLE 1

Low-voltage cables, both according to the present invention and not,were prepared according to the cable design shown in FIG. 1.

The cable conductor 1 was made of copper and had a cross section ofabout 1.5 mm².

Main extruder size: 150/26D Tip die: 1.38 mm Ring die: 2.70 mm Foamingmb dosing Maguire (gravimetric type) system: Temperature Profile (° C.):Z1 Z2 Z3 Z4 Z5 Z6 H1 H2 H3 H4 160 180 190 200 210 220 220 230 240 240Line speed: 1500 m/min Main extruder speed: 48 rpm current: 65 Apressure: 380 bar Hot cable diameter: 2.9 mm Cold cable diameter: 2.9 mm

The thickness of each insulating coating was about 0.6 mm. 0.7 mm inaccordance with Italian Standard CEI-UNEL 35752 (2nd Edition—February1990).

Each cable was subsequently cooled in water and wound on a storage reel.

Table 1 also set forth the expansion degrees of each polymeric blend.

TABLE 1 Crosslinking Foaming agent Expansion system % Density DegreeCable Polyolefin Kind Mol Kind w/w (g/cm³) (%) 1 LL4004 EL Sil/perox0.01 — — 0.926 0.0 2 LL4004 EL Sil/perox 0.01 Hostatron 0.27 0.628 32.23 BPD 3220 Silfin 06 0.006 — — 0.903 0.0 4 BPD 3220 Silfin 06 0.006Hostatron 0.24 0.700 22.2 5 BPD 3220 Silfin 06 0.006 Hostatron 0.150.860 4.4 6 BPD 3220 Silfin 06 0.008 Hostatron 0.15 0.850 5.6 7 BPD 3220Silfin 06 0.006 Hostatron 50% 0.15 0.817 9.5 8 BPD 3220 Silfin 06 0.006Hostatron 50% 0.18 0.764 15.4 9 BPD 3220 Silfin 06 0.006 Hostatron 0.180.787 12.8 10  BPD 3220 Sil/perox 0.006 Hostatron 0.24 0.711 21.5 11*BPD 3220 Sil/perox 0.12 Hostatron 0.09 0.906 0.3 12  BPD 3220 Sil/perox0.12 Hostatron 0.18 0.833 8.1 13  BPD 3220 Sil/perox 0.12 Hostatron 0.240.694 23.4 14* BPD 3220 Sil/perox 0.006 Hostatron 50% 0.60 0.481 48.015* LL4004 EL Sil/perox 0.01 Hydrocerol 0.40 0.611 34.0 16* BPD 3220Silfin 06 0.006 Hydrocerol 0.16 0.876 3.0 17* BPD 3220 Silfin 06 0.006Hydrocerol 0.45 0.570 15.4 18  BPD 3220 Sil/perox 0.006 Hostatron 50%0.24 0.764 38.0 N.B. - the mol and % w/w refer to the content of,respectively, silane or foaming agent The cables marked with an asteriskare comparative ones. LL 4004 EL = LLDPE with an MFL of 0.33 g/10 min at190° C. under a load of 2.16 kg (by ExxonMobil Chemical) BPD 3220 =LLDPE (by BP) Sil/perox = LUPEROX 801 (by Arkema) plus DYNASYLAN VTMO(by Degussa) Silfin 06 = mixture of vinylsilane, peroxide initiator andcatalyst for crosslinking (by Degussa) Hostatron = PV22167 foamingsystem based on azodicarbonamide foaming agent (by Clariant) Hostatron50% = PV22167 foaming system based on azodicarbonamide foaming agent (byClariant) at 50% in EVA masterbatch Hydrocerol = BIH 40, foaming systembased on a mixture of citric acid and basic sodium carbonate as foamingagents (by Clariant). The composition of said blends is shown in Table 1(expressed in parts by weight per 100 parts by weight of base polymer).The % w/w of the foaming agent refers to the amount of foaming agentadded. Cables 1 and 3 (no foaming agent used) are provided as referencefor calculating the expansion degree, and for the electrical testing thecables with the crosslinked and expanded insulating layer. Cables15*-17* relates are insulated by polymeric blends expanded with anendothermic foaming agent (Hydrocerol) Cables 11* and 14* are insulatedby polymeric blends expanded with an exothermic foaming agent in anamount out of the preferred range. In the case of Cable 11, theexpansion degree is substantially null, thus this cable is not endowedwith advantages in term of flexibility and peel-off capacity withrespect to a cable having a non-expanded insulating coating. On theother side, Cable 14 shows an insulating coating with an expansiondegree too high and impairing the mechanical properties, as it will beshown in the Example 3.

EXAMPLE 2

Cables as from example 1 were tested to evaluate the cross-linkingdegree of the insulating coating thereof, according to the Italianstandard regulation CEI EN 60811-2-1:1999-05. The results are set forthin Table 2.

TABLE 2 Expansion Hot set Cable Density (g/cm³) Degree (%) Elongation(%) 1 0.926 0.0 45 2 0.628 32.2 50 3 0.903 0.0 90 4 0.700 22.2 110 50.860 4.4 75 6 0.850 5.6 85 8 0.764 15.4 100 9 0.787 12.8 90 10  0.71121.5 107 12  0.833 8.1 35 13  0.694 23.4 45 14* 0.481 48.0 110 15* 0.61134.0 60 16* 0.876 3.0 >200 17* 0.764 15.4 broken 18  0.570 38.0 50 Thecables marked with an asterisk are comparative ones. Taking into accountthat the limit prescribed by the above mentioned requirement is up to175%, Cable 16* shown to be out of scale, i.e. the polyolefin did notcross-link sufficiently and this negatively affects the thermopressureresistance. Cable 17* broke due to an excessive average cell diameterand to an irregular cell distribution in the expanded polyolefin, asshown in FIG. 2. The two failures reported in Table 2 is ascribed to theuse of an endothermic foaming agent as the sole foaming agent of theprocess for producing a cross-linked and expanded polyolefin material.The endothermic foaming agent could negatively interact with thesilane-based cross-linking system.

EXAMPLE 3

Cables produced as from example 1 were tested in order to measure themechanical properties thereof, according to the Italian standardregulation CEI EN 60811-1-1:2001-06, requiring a tensile strength of atleast 12.5 MPa. The results are set forth in Table 3.

TABLE 3 Expansion Tensile Strength Cable Density (g/cm³) Degree (%) MPa1 0.926 0.0 20.00 2 0.628 32.2 12.50 3 0.903 0.0 20.54 4 0.700 22.213.57 5 0.860 4.4 17.37 6 0.850 5.6 18.92 8 0.764 15.4 16.43 9 0.78712.8 17.02 10  0.711 21.5 18.90 12  0.833 8.1 18.10 13  0.694 23.4 14.1014* 0.481 48.0 9.70 15* 0.611 34.0 9.20 18  0.570 38.0 12.80 The cablesmarked with an asterisk are comparative ones. Cable 14* insulated by apolymeric blends expanded with an exothermic foaming agent according tothe invention but in an amount out (higher) of the selected range, andproviding an insulating coating with an expansion degree (48.0%) notaccording to the invention. Such cable showed unsuitable mechanicalfeatures. Cable 15* insulated by a polymeric blends expanded with anendothermic foaming agent and provided with an insulating coating havingan expansion degree in the range of the invention (34.0%) showed anywaypoor mechanical features. This is due to the use of an endothermicfoaming agent that yield an expansion degree unsatisfactory from thequalitatively point of view.

EXAMPLE 4

In the following Table 4 the mechanical properties and the hot set oftwo cables according to the invention and one comparative cable wereevaluated together with the average cell diameter.

The average cell diameter was evaluated as follows. An expanded portionof insulating coating was randomly selected and cut perpendicularly tothe longitudinal axis. The cut surface was observed by a microscope andthe image was formed on a photograph. The major diameter (taking intoaccount that the cells can be not perfectly round) of 50 randomlyselected cells was measured. The arithmetic mean of the 50 measureddiameters represents the average cell diameter.

For each cable two samples were tested. All of the cables differed fromthose of the previous examples just in that conductor 1 had a crosssection of about 2.5 mm².

The insulation coatings for cables 17* and 19 were extruded with aDDR=1, the insulation coating for cable 20 was extruded with a DDR=0.7.

The draw down ratio was calculated by comparing the cross sectional areaof the die to the cross sectional area of the extrusion. The followingformula was applied:

${D\; D\; R} = \frac{D_{d}^{2} - D_{m}^{2}}{D_{t}^{2} - D_{b}^{2}}$wherein DDR=draw down ratioD_(d)=Internal diameter of extrusion ring-dieD_(m)=External diameter of the tip-dieD_(t)=External diameter tubeD_(b)=Internal diameter tube.

TABLE 4 Mechanical Expansion Average cell properties Hot set Foamingagent degree diameter TS EB Elongation Cable Polyolefin Kind % w/w (%)μm (MPa) (%) (%)  17* BPD Hydrocerol 0.24 15.4 500 11.03 486.5 both 3220broken 19 BPD Hostatron 0.18 13 300 15.61 580.6 90; 100 3220 50% 20 BPDHostatron 0.18 13 100 17.15 573.3 80; 80 3220 50% TS = Tensile strengthEB = Elongation at break The cables marked with an asterisk arecomparative ones. The decreasing of the average cell diameter was foundto improve the mechanical characteristics, such as hot set and tensilestrength, of the insulating layer. Cable 17* insulation have anexpansion degree similar to that of the cables of the invention, but theaverage cell diameter is higher. The high average cell diameter of cable17* is accompanied by an uneven e expansion, as visible in FIG. 2.Cables 19 and 20 according to the invention have improved mechanicalproperties with respect of the comparative Cable 17*. In particular,Cable 20 has the same expansion degree of Cable 19, but a lower averagecell diameter due to the lower extrusion DDR and is endowed with asuperior tensile strength. Said cables are shown in FIGS. 3 and 4,respectively.

EXAMPLE 5

A cables as from example 4 was tested in order to measure the ease ofpeeling-off the insulating coating material from the conductor, incomparison with an unexpanded cable 3.

Six 120 mm-long samples for each cable were provided. Each sample waspreviously peeled-off to an extent of 40 mm, so as 80 mm of sample wereemployed in the test, effected according to MIL-W-22759

The results are set forth in the following Table 5.

TABLE 5 peeling-off (sfilability test) Expansion max min average CableDegree (%) load (N) load (N) load (N) 3 — 53.27 23.02 38.14 20 13 16.2110.73 13.47 The force applied for peeling off the cable of the inventionis lower than that for the reference cable 3 having an insulating layernot expanded. The max load is the force applied for starting thepeeling-off.

EXAMPLE 6

Three cables produced according to Example 1 and sheathed with PVCcontaining decyl phthalate as plasticizer (sheath thickness=1.56 mm)were tested to evaluate the mechanical characteristics thereof after 7days at 100° C. (ageing test according to EN 60811). According to thetest requirement the maximum variation of the tensile strength must notexcess ±25%. The results are set forth in Table 6.

TABLE 6 Mechanical characteristic Expansion Tensile Maximum Densitystrength Variation Cable (g/cm³) Degree (%) (MPa) (%) 3 0.903 0.0 19.72± 0.49 −25.3 ± 2.6 4 0.700 22.2 12.25 ± 0.63 −12.2 ± 6.4 5 0.860 4.417.72 ± 1.41  12.4 ± 4.9 6 0.850 5.6 18.91 ± 0.79 −12.4 ± 5.2 Cables 4-6according to the invention passed the test, whereas reference cable 3having an insulating layer not expanded did not.

The presence of an expanded insulating layer improves the mechanicalproperties after the compatibility test, decreasing the negative effectsof the migration of the plasticizer present in the cable sheath.

The invention claimed is:
 1. An electric cable comprising at least onecore consisting of a conductor and an insulating coating surroundingsaid conductor and in contact therewith, said insulating coatingconsisting of a layer of expanded, silane-crosslinked polyolefinmaterial having an expansion degree of 3% to 40%, wherein saidinsulating coating has an average cell diameter equal to or lower than300 μm.
 2. The electric cable according to claim 1, which is a lowvoltage cable.
 3. The electric cable according to claim 1, comprisingthree cores.
 4. The electric cable according to claim 1, wherein thepolyolefin material is selected from polyolefins, copolymers of olefins,olefins/unsaturated esters copolymers, polyesters, and mixtures thereof.5. The electric cable according to claim 4, wherein the polyolefinmaterial is selected from low-density polyethylene, medium-densitypolyethylene, high-density polyethylene, linear low-densitypolyethylene, ethylene-propylene elastomeric copolymers,ethylene-propylene-diene terpolymers, ethylene/vinyl ester copolymers,ethylene/acrylate copolymers, ethylene/α-olefin thermoplasticcopolymers, and copolymers or mechanical blends thereof.
 6. The electriccable according to claim 5, wherein the polyolefin material is selectedfrom low-density polyethylene, medium-density polyethylene, high-densitypolyethylene, linear low-density polyethylene, and a blend thereof withethylene-propylene-diene terpolymers or olefin copolymers.
 7. Theelectric cable according to claim 6, wherein the polyolefin material isselected from linear low-density polyethylene and the blend thereof withethylene-propylene-diene terpolymers or olefin copolymers.
 8. Theelectric cable according to claim 6, wherein the polyolefin material isa blend of a polyethylene material and a copolymer material, thecopolymer material being present in an amount of from 5 phr to 30 phr.9. The electric cable according to claim 1, wherein the insulatingcoating has an expansion degree of 5% to 30%.
 10. The electric cableaccording to claim 9, wherein the insulating coating has an expansiondegree of 10% to 25%.
 11. The electric cable according to claim 1,wherein the insulating coating has an average cell diameter equal to orlower than 100 μm.
 12. The electric cable according to claim 1, whereina circumferential portion of the expanded insulating coating contactingthe conductor is not expanded.
 13. The electric cable according to claim1, comprising a sheath layer, in radially external position with respectto the insulating layer.
 14. A method for improving the ageing stabilityof a cable comprising applying to a conductor, an insulating layer and asheath, said insulating coating consisting of a silane-cross-linkedpolyolefin material having an expansion degree of 3% to 40%, whereinsaid insulating coating has an average cell diameter equal to or lowerthan 300 μm.
 15. A process for manufacturing an electric cablecomprising at least one core consisting of a conductor and an insulatingcoating consisting of a layer of expanded, silane-crosslinked polyolefinmaterial surrounding said conductor, comprising the steps of: providinga polyolefin material, a silane-based cross-linking system and a foamingsystem comprising at least one exothermic foaming agent in an amount of0.1% to 0.5% by weight with respect to the total weight of thepolyolefin material; forming a blend with the polyolefin material, thesilane-based cross-linking system and the foaming system; extruding theblend on the conductor to form the insulating coating; and wherein saidinsulating coating has an average cell diameter equal to or lower than300 μm.
 16. The process according to claim 15, wherein the polyolefinmaterial is selected from polyolefins, copolymers of olefins,olefins/unsaturated ester copolymers, polyesters, and mixtures thereof.17. The process according to claim 15, wherein the polyolefin materialis selected from low-density polyethylene, medium-density polyethylene,high-density polyethylene, linear low-density polyethylene,ethylene-propylene elastomeric copolymers, ethylene-propylene-dieneterpolymers, ethylene/vinyl ester copolymers, ethylene/acrylatecopolymers, ethylene/α-olefin thermoplastic copolymers, and thecopolymers or mechanical blends thereof.
 18. The process according toclaim 17, wherein the polyolefin material is selected from low-densitypolyethylene, medium-density polyethylene, high-density polyethylene,linear low-density polyethylene, and a blend thereof withethylene-propylene-diene terpolymers or olefin copolymers.
 19. Theprocess according to claim 18, wherein the polyolefin material isselected from linear low-density polyethylene and a blend thereof withethylene-propylene-diene terpolymers or olefin copolymers.
 20. Theprocess according to claim 15, wherein the silane-based cross-linkingsystem comprises at least one silane selected from (C₁-C₄)alkyloxysilanes with at least one double bond.
 21. The process according toclaim 20, wherein the at least one silane is selected from vinyl- andacryl-(C₁-C₄)alkyloxy silanes.
 22. The process according to claim 21,wherein the at least one silane is selected fromγ-methacryloxy-propyltrimethoxy silane, vinyltrimethoxysilane,vinyltriethoxysilane, vinyldimethoxyethoxysilane,vinyltris-(2-methoxyethoxy)silane, and mixtures thereof.
 23. The processaccording to claim 15, wherein the silane-based cross-linking systemcomprises at least one peroxide.
 24. The process according to claim 23,wherein the at least one peroxide is selected fromdi(terbutylperoxypropyl-(2)-benzene, dicumyl peroxide, diterbutylperoxide, benzoyl peroxide, terbutylcumyl peroxide,1,1-di(ter-butylperoxy)-3,3,5-trimethyl-cyclohexane,2,5-bis(terbutylperoxy)-2,5-dimethylhexane,2,5-bis(terbutylperoxy)-2,5-dimethylhexineterbutylperoxy-3,5,5-trimethylhexanoate, ethyl3,3-di(terbutylperoxy)butyrate, butyl-4,4-di(terbutylperoxy)valerate,and terbutylperoxybenzoate.
 25. The process according to claim 15,wherein the silane-based cross-linking system comprises at least onecross-linking catalyst.
 26. The process according to claim 25, whereinthe at least one cross-linking catalyst is selected from an organictitanate and a metallic carboxylate.
 27. The process according to claim26, wherein the at least one cross-linking catalyst is dibutyltindilaurate.
 28. The process according to claim 15, wherein the silanecross-linking system is added in an amount sufficient to provide theblend with 0.003 to 0.015 mol of silane per 100 grams of polyolefinmaterial.
 29. The process according to claim 28, wherein the silanecross-linking system is added in an amount sufficient to provide theblend with 0.006 to 0.010 mol of silane per 100 grams of polyolefinmaterial.
 30. The process according to claim 15, wherein the foamingsystem comprises at least one endothermic foaming agent.
 31. The processaccording to claim 20, wherein the at least one endothermic foamingagent is in an amount equal to or lower than 20% by weight with respectto the total weight of the polyolefin material.
 32. The processaccording to claim 15, wherein the exothermic foaming agent is an azocompound.
 33. The process according to claim 32, wherein the azocompound is selected from azodicarbonamide, azobisisobutyronitrile, anddiazoaminobenzene.
 34. The process according to claim 33, wherein theazo compound is azodicarbonamide.
 35. The process according to claim 15,wherein the exothermic foaming agent is in an amount of 0.15% to 0.24%by weight with respect to the total weight of the polyolefin material.36. The process according to claim 15, wherein the foaming system isadded to the polyolefin material as a masterbatch comprising polymermaterial.
 37. The process according to claim 36, wherein the polymermaterial masterbatch is selected from an ethylene homopolymer and anethylene copolymer.
 38. The process according to claim 37, wherein thepolymer material masterbatch is selected from ethylene/vinyl acetatecopolymer, ethylene-propylene copolymer and ethylene/butyl acrylatecopolymer.
 39. The process according to claim 36, wherein themasterbatch comprises 1% by weight to 80% of foaming agent by weightwith respect to the total weight of the polymer material.
 40. Theprocess according to claim 39, wherein the masterbatch comprises 5% byweight to 50% by weight of foaming agent with respect to the totalweight of the polymer material.
 41. The process according to claim 40,wherein the masterbatch comprises 10% by weight to 40% by weight offoaming agent with respect to the total weight of the polymer material.42. The process according to claim 15, wherein the foaming systemcomprises at least one activator.
 43. The process according to claim 42,wherein the at least one activator is selected from transition metalcompounds.
 44. The process according to claim 15, wherein the foamingsystem comprises at least one nucleating agent.
 45. The processaccording to claim 44, wherein the at least one nucleating agent is anactive nucleator.
 46. The process according to claim 15, wherein thestep of forming a blend with the polyolefin material, the silane-basedcross-linking system and the foaming system is effected in a singlescrew extruder.
 47. The process according to claim 46, wherein theextruder is fed by a multi component dosing system of volumetric type.48. The process according to claim 15, wherein the step of forming ablend with the polyolefin material, the silane-based cross-linkingsystem and the foaming system is preceded by a step of off-line mixingthe polyolefin material, the silane-based cross-linking system and thefoaming system.
 49. The process according to claim 15, wherein the stepof extruding the blend on the conductor for providing said conductorwith an insulating coating comprises the steps of: feeding saidconductor to an extruding machine; and depositing the insulating layerby extrusion.
 50. The process according to claim 15, wherein the step ofextruding the blend is effected by means of a die with a draw down ratiolower than
 1. 51. The process according to claim 50, wherein the drawdown ratio is lower than 0.9.
 52. The process according to claim 51,wherein the draw down ratio is lower than 0.8.
 53. The process accordingto claim 15, comprising the step of extruding a sheath layer in aradially circumferential external position with respect to the at leastone conductor coated with the relevant insulating coating.